Alignment guide

ABSTRACT

An orthopaedic joint prosthesis assembly includes a head part of an orthopaedic joint prosthesis component and an alignment guide. The head part has a spherical bearing. surface for articulation with a corresponding joint surface, and an assembly surface having a first bore formed in it for receiving a spigot on another part of the orthopaedic joint prosthesis, The bore has a first axis that extends perpendicular to the assembly surface. There is a discontinuity at an interface between the bearing surface and the assembly surface, and the assembly surface is arranged on a plane which is parallel to, or contains, a plane which is defined by the opening to the bore in the head part when the head part is viewed from one side in cross-section. The alignment guide has an axial portion and an arm extending from the axial portion. The arm includes as distal portion which is configured to engage the assembly surface of the head part, with the axial portion defining a second axis and being arranged to engage the bearing surface of the head part when the head part is mounted within the guide such an impaction force applied along the second axis is directed along the first axis.

BACKGROUND TO THE INVENTION Field of the Invention

This invention relates an orthopaedic joint prosthesis assembly whichincludes a head part of a joint prosthesis component and alignmentguide.

Description of the Related Art

Many orthopaedic joint prosthesis components are modular. A modularconstruction allows components to be assembled to meet particularrequirements, for example to take account of anatomical variationsbetween patients or surgeon preference. Examples of modular orthopaedicjoint prosthesis components include femoral components of hip jointprostheses and humeral components of shoulder joint prostheses. Each ofthese comprises a stem part which is fitted in the intramedullary cavityof the bone (femur or humerus) and a head part. The head pan has abearing surface for articulation with a corresponding joint surfacewhich can be provided by a mating joint prosthesis component (anacetabular component or a glenoid component) or by the patient's naturaltissue. The head part has an assembly surface opposite to the bearingsurface where the head part is connected to the stem part.

It is common to fasten modular parts of a joint prosthesis component toone another by means of a self-locking taper in which a tapered spigot(a term used to refer to a short projection on one component which fitsinto a bore in another component, in order to fasten the componentstogether) on one part is received in a correspondingly tapered bore inthe other part. An example of such a self-locking taper is a Morse taperin which the angle between the tapered surface of each of the spigot andthe bore and the longitudinal axis of the spigot and the bore (when thepart is viewed in cross-section) is about 1.4° to about 1.5°.

The security of a joint between two taper locked parts depends on theforce that is applied to the parts when they are assembled. Sufficientforce should be applied to ensure proper engagement of the surfaces ofthe spigot and the bore. However, it can sometimes be important toensure that the applied force does not exceed a maximum limit, forexample to avoid damage to a patient's bone tissue if the parts arebeing assembled with one implanted in the patient's bone, or to avoiddamage to the parts of the component.

EP-A-1707160 discloses a device for applying an assembly force to partsof an orthopaedic joint prosthesis component. The device includes ahollow housing and an impacting rod which extends from the housing witha tip for contacting one of the parts of the prosthesis component. Thehousing contains a piston which can slide in the housing. A spa in islocated be an end a the piston and a closed end of the housing. Use ofthe device involves compressing the spring by forcing the piston towardsthe closed end of the housing and then releasing the piston so that itcan slide within the housing, acted on by the spring as it relaxes.

SUMMARY OF THE INVENTION

The present invention provides an orthopaedic joint prosthesis assemblythat includes an alignment guide which can engage an assembly surface ofthe head part of an orthopaedic joint prosthesis, opposite its bearingsurface, so that an assembly force is applied to the parts of theprosthesis component can be directed along a first axis defined by thebore in the head part through use of the assembly surface as a referencefor the alignment guide.

The invention therefore provides an orthopaedic joint prosthesisassembly which comprises:

a. a head part of an orthopaedic joint prosthesis component, which has aspherical bearing surface for articulation with a corresponding jointsurface, and an assembly surface having a first bore formed in it forreceiving a spigot on another part of the orthopaedic joint prosthesis,said bore having a first axis that extends perpendicular to the assemblysurface, and in which there is a discontinuity at an interface be thebearing surface and the assembly surface, the said assembly surfacebeing arranged on a plane which is parallel to, or contains, a planewhich is defined by the opening to the bore, in the head part when thehead part is viewed from one side in cross-section, and

b. an alignment guide comprising an axial portion and an arm extendingfrom the axial portion, the arm including a distal portion and aproximal portion, the distal portion being configured to engage theassembly surface of the head part, the axial portion defining a secondaxis and being arranged to engage the bearing surface of the head part,directly or indirectly, when the head part is mounted within the guide,with the second axis coincident with the first axis, such when animpaction force is applied along the second, axis the force is directedalong the first axis.

The orthopaedic joint prosthesis assembly of the invention can be usedin a surgical procedure to implant an orthopaedic joint prosthesiscomponent which includes a head part and another part. Frequently, theother part will be a stem part which can be fitted in the intramedullarycavity of a patient's long bone. The invention is therefore useful in asurgical procedure to implant a femoral component of a hip jointprosthesis in which the bore in the assembly surface of the head partreceives a spigot on the stem part which is intended to be fitted intothe intramedullary cavity of the patient's femur. The invention is alsouseful in a surgical procedure to implant a humeral component of ashoulder joint prosthesis in which the bore in the assembly surface orthe head part receives a spigot on the stem part which is intended to befitted into the intramedullary cavity of the patient's humerus.

The alignment guide will be dimensioned to suit the dimensions of theprosthesis head part which is to be manipulated. The transversedimension (which will be the diameter when the head pad is configuredwith a spherical bearing surface) of the head part of a femoralcomponent of a hip joint prosthesis will frequently be at least about 15mm. It could be up to about 50 mm. The transverse dimension of the headpart of a humeral component of a shoulder joint prosthesis willfrequently be at least about 25 mm. It could be up to about 60 mm.

Each of the bearing surface and the assembly surface of the head part ofthe prosthesis component of the joint prosthesis assembly can becylindrically symmetrical, with the axis of the bore is coincident withthe axis of symmetry of the bearing surface. The term “cylindricallysymmetrical” is used to refer to a shape which is rotationallysymmetrical of infinite order. However, the joint prosthesis assemblycan include a head part in which one or both of the bearing surface andthe assembly surface is not cylindrically symmetrical. For example, anassembly surface might be generally circular in outline with the bore inthe assembly surface being offset relative to the centre of the circularoutline. A bearing surface might be defined by the surface of part of asphere. However, a bearing surface might be curved (convex or concave)but not necessarily part spherical.

The bearing surface on the head part can be convex. This will generallybe the case when the prosthesis component is a femoral component of ahip joint prosthesis. It will also generally be the case when theprosthesis component is a humeral component of an anatomic shoulderjoint prosthesis.

The bearing surface on the head part can be concave. This will generallybe the case when the prosthesis component is a humeral component of areverse shoulder joint prosthesis.

The axis which is defined by the bore in the head part extendsperpendicular to the assembly surface. The assembly surface is at oneend of the axis and the centre of the bearing surface is at the oppositeend of the axis. The assembly surface surrounds the bore. The assemblysurface will generally be contained in a plane which is perpendicular tothe axis defined by the bore in the head part. The assembly surface isarranged on a plane which is parallel to and/or contains the opening tothe bore in the head part. The assembly surface can be shaped as anarrow ridge. The assembly surface can be provided in the form of one ormore generally flat portions which are defined by a straight lines whenthe head part is viewed in cross-section. At least one flat portion canbe planar, containing or parallel to the plane of the opening to thebore in the head part. The assembly surface can includes a chamferportion extending around the head part which is inclined to the planewhich is defined by the opening to the bore in the head part when thehead part is viewed from one side in cross-section. The assembly surfacecan include a flat planar portion which lies in the plane defined by theopening to the bore in the head part, and a chamfer portion between theflat planar portion and the bearing surface of the head part. Theassembly surface can include a portion which is curved when the headpart is viewed from one side in cross-section. For example, the assemblysurface can include a portion which is convex and/or a portion which isconcave. Such portions (flat, chamfer and curved) can extend annularlyaround the opening to the bore in the head part.

The assembly surface will be capable of being distinguished from theadjacent bearing surface. The bearing surface itself will be free ofdiscontinuities which might interfere with smooth articulation of thehead part with a corresponding joint surface as is the case when, forexample, the bearing surface is the surface of part of a sphere. It willfrequently be the case that there will be a discontinuity at theinterface between the bearing and assembly surfaces, for example througha change in curvature which gives rise to a discernible ridge. Thebearing surface will frequently be polished to a lower surface roughnessthan the assembly surface. Appropriate surface roughness levels for thebearing surface of an orthopaedic joint prosthesis component are wellestablished. The assembly surface might have markings on it, for exampleto identify the component (for example its size).

The orthopaedic joint prosthesis assembly can include a part of theorthopaedic joint prosthesis having a spigot which can be received inthe bore in the assembly surface of the head part. An example of such apart is a stem part of an orthopaedic joint prosthesis which can befitted at least partially into the intramedullary cavity of a patient'sbone.

In the constructions of the orthopaedic joint prosthesis assembly inwhich the axial portion of the alignment guide is arranged to directlycontact the bearing surface of the head part, the axial portion includesa bearing surface seating member.

Optionally, the axial portion is a shaft having a distal end and aproximal end. A longitudinal axis extends between the distal end and theproximal end of the shaft. This longitudinal axis defines the secondaxis. The distal end of the shaft includes a bearing surface sealingmember configured to directly engage the bearing surface of theprosthesis component head part. The second axis extends centrallythrough the bearing surface seating member. An impaction force applieddirectly to the proximal end of the shaft is therefore aligned with thefirst axis that is defined by the bore in the head part.

In some constructions, the shaft includes a blind-bore extendinglongitudinally from the proximal end towards the distal end. Theblind-bore is dimensioned to receive an impaction tool, such as animpaction rod (through which an impaction force can be applied to theprosthesis component head part). The impaction rod has a longitudinalaxis. When the impaction rod is received within the blind bore thelongitudinal axis of the impaction rod is coincident with the secondaxis. An impaction force can be applied to head part of the prosthesiscomponent by applying an impaction force to the proximal end of theimpaction rod. An impaction force applied indirectly to the shaft inthis manner is therefore aligned with the first axis that is defined bythe bore in the head part.

Optionally, the axial portion is a hemi-spherical hub having a convexouter surface and a concave inner surface. The convex outer surface hasa pole. An axis extends through the pole. This pole axis defines thesecond axis. The second axis is coincident with the first axis asdefined by the bore in the head part. The concave inner surface definesa space in which a head part can be received and is configured todirectly engage the bearing surface of the prosthesis component headpart. An impaction rod (through which an impaction force can be appliedto the prosthesis component head part) is connectable to the hub at thepole. The impaction rod has a longitudinal axis. When the impaction rodis connected to the hub at the pole, the longitudinal axis of theimpaction rod is coincident with the second axis. An impaction forceapplied directly to the impaction rod is therefore aligned with thefirst axis that is defined by the bore in the head part.

The impaction rod may be connected to the hub via a socket located atthe pole. The socket may be dimensioned to receive the impaction rod. Insome other constructions, a sleeve can be received within the socket.The sleeve has a longitudinal axis that is coincident with the secondaxis. An impaction rod can extend through the sleeve component and is asliding fit therein. The longitudinal axis of the impaction rod is alsoaligned with the second axis. The distal end of the impaction rod canform at least part of the bearing surface sealing member. An impactionforce applied via an impaction rod in this manner is therefore alignedwith the first axis that is defined by the bore in the head part.

In other constructions of the orthopaedic joint prosthesis assembly inwhich the axial portion of the alignment guide is arranged to indirectlycontact the bearing surface of the head part, the bearing surfaceseating member is provided on a different component of the assembly fromthe axial portion.

Optionally, the axial portion is a sleeve having a through bore. Thesleeve has a proximal end and a distal end. A longitudinal axis extendsbetween the distal end and the proximal end of the sleeve. Thislongitudinal axis defines the second axis.

A shaft is received in a sliding tight fit within the through bore ofthe sleeve. The shaft has a proximal end and a distal end. Alongitudinal axis extends between the distal end and the proximal end ofthe shaft. This shaft axis is aligned with the second axis, as definedby the sleeve. The distal end of the shaft is configured as a bearingsurface seating member and contacts the bearing surface of the head partwhen the head part is mounted within the guide.

In some on the shaft is an impaction tool, such as an impaction rod(through which an impaction force can be applied to the prosthesiscomponent head part). The distal end of the impaction rod is configuredas a bearing surface seating member.

In some constructions, the shaft includes a blind-bore extending fromthe proximal end towards the distal end. The blind-bore is configured toreceive an impaction tool, such as an impaction rod. An impaction forcecan be applied directly to the proximal end of the shaft (for exampleusing an instrument such as a hammer or a mallet, or an instrument whichgenerates a controlled impaction force such as the instrument disclosedin EP-A-707160) or indirectly via an impaction rod received within theblind-bore.

Optionally, the shaft can be translated (for example, to be driven oradvanced) within the sleeve along the second axis as defined by thesleeve. For example, the sleeve and the shaft can have cooperatingthreads so that the shaft can be advanced through the sleeve by rotatingit about its axis. For example, the shaft can have an external threadwhich threadingly engages an internally threaded sleeve.

This ability of the shaft to be translated allows the distance betweenthe bearing surface seating member and the assembly surface seatingmember to be adjusted so that the head part of the prosthesis componentcan be held within the alignment guide between the two seating members.This can facilitate assembly of the head and stem (or other) parts ofthe prosthesis component. Rotating the shaft relative to the sleeve canincrease the distance between the seating members, facilitating removalof the head part from the space between the seating members. It alsoenables the alignment guide to be configured for use with head partshaving a range of different sizes.

It will frequently be preferred that the surface of the bearing surfaceseating member which contacts the bearing surface of the head part ofthe prosthesis component is configured so that its shape iscomplementary to that of the bearing surface of the head part. Thebearing surface of the head part can then be a nesting fit with thecontact surface of the bearing surface seating member. For example, whenthe bearing surface of the head part is convex, it can be appropriatefor the surface of the bearing surface seating member which contacts thebearing surface to be concave. When the bearing surface of the head partis concave, it can be appropriate for the surface of the bearing surfaceseating member which contacts the bearing surface to be convex.

The surface of the bearing surface seating member which contacts thebearing surface of the head part of the prosthesis component should beprovided by a material and finished in such a way that the risk ofdamage (for example, by scratching) to the bearing surface of the headpart is minimised. The contact surface could be provided by a materialwhich is softer than the material of the bearing surface. Suitablematerials are known from their use in instruments which are used tocontact a bearing surface of an orthopaedic joint prosthesis whenassembling or implanting it. Examples of suitable materials include lowand high density polyethylenes, certain silicone elastomers, and certainpoly(phenyl sulphones) (such as the material sold under the trade markRadel).

The bearing surface seating member can be configured so that it isengaged by the head part of the prosthesis component with a press fit.This can be achieved by providing the bearing surface seating memberwith opposing portions which extend beyond the widest point on the headpart. For example, the bearing surface seating member can be made from aresiliently deformable flexible material in the form of a concave recesswhose wall is required to flex outwardly in order to insert a head partinto the recess. The resiliently deformable characteristics of thematerial of the bearing surface seating member can mean that the seatingmember springs back once a head part has been positioned within thealignment guide to grip the head part.

The bearing surface seating member can have a plurality of radiallyextending fingers (for example at least two fingers, or at least threefingers) which are shaped to fit closely against the bearing surface ofthe head part. The fingers can help to locate the head part centrallyrelative to the axial portion through which an impaction force isapplied to the head part, and to position it so that it is properlyaligned with the second axis as defined, by the axial portion.Optionally, the fingers can be sufficiently long to extend beyond thewidest point of the head part. The widest point might be the equator inthe case of the head part of a femoral component of a hip jointprosthesis, or it might be the interface between the bearing andassembly surfaces of a humeral component of a shoulder joint prosthesis.It can then be preferred that the fingers are made from a resilientlyflexible material. The fingers can then be help to retain the head partof the prosthesis component within the space between the bearing surfaceand assembly surface seating members.

Radially extending fingers can extend radially from a point which lieson the axis defined by the bore in the head part. Radially extendingfingers can extend radially from a connection with the axial portionthrough which an impaction force is applied to the head part.

It can be convenient for the bearing surface seating member to becapable of being removably connectable to the component of the alignmentguide on which it is provided. This allows a bearing surface seatingmember to be replaced, for example in order to select one which isadapted for use with a different head part, or because a bearing surfaceseating member is damaged. The components might be connected to oneanother by means of mating threads. For example, in constructions inwhich the bearing surface seating member is provided at the distal endof a shaft, the end of the shaft could have an external thread and thebearing surface seating member can have a bore formed in it with aninternal thread.

The arm which extends from the axial portion of the alignment guidedefines a space around at least part of the periphery of the head partof the joint prosthesis.

The alignment guide can include more than one arm, for example at leasttwo arms, or at least three arms, or at least four arms. Optionally,there can be spaces between neighbouring arms which allow a head partmounted within the alignment guide to be viewed and inspected.

Optionally, the at least one arm is capable of being pivoted outwardlyto allow access for the head part to be mounted within the alignmentguide. The at least one arm can be pivotally connected to the axialportion at or towards one end. The at least one arm can be pivotedoutwardly to allow access for the head part to be mounted within thealignment guide. The at least one arm can them be pivoted inwardly toposition the assembly surface seating member in contact with theassembly surface of the head part.

An alignment guide which has at least one pivoting arm can be used withhead parts which have different sizes.

Optionally, the axial portion includes first and second arms which areconnected to the axial portion at its widest point so that they can bepivoted relative to the axial portion between a retracted position whichallows a head part to be located within the said space and a deployedposition in which a head part position in the said space is retainedtherein.

When the alignment guide includes first and second pivotably mountedarms, a shaft which translates relative to the axial portion of thealignment guide can have a camming surface which engages respectivecamming surfaces on internal surfaces of the arms, causing the arms tobe pivotably displaced outwardly when the shaft is translated in adistal direction. The camming surface on the shaft can be provided bythe surface of a portion of the shaft which is flared outwardly. Thiscan act on an appropriately located portion of the internal surface ofeach of the arms. For example, the camming surface in each of the armscan be provided by an inwardly protruding tab on the internal surfacesof the arms. The outward displacement of the camming surfaces candisengage the distal portion of each arm from the assembly surface ofthe head part as the head part and the stem or other part of theprosthesis component are assembled.

An alignment guide which has two or more pivoting arms can be engagedwith a head part and then used to move or otherwise manipulate the headpart. This can be particularly useful when the head part is beingmanipulated during preparatory steps prior to a surgical procedure. Forexample, the alignment guide can be engaged with a head part which ispresented in its packaging, and then used to remove the head part fromthe packaging. The alignment guide can be used to position the head parton a spigot on a stem part. These steps can be performed without anyneed to touch the head part. This can help to preserve a polished finishon the bearing surface of the head part.

The at least one arm should have sufficient rigidity to ensure that itis not deformed unacceptably when in use.

The at least one arm can have openings which allow the head part to beinspected when it is mounted within the alignment guide. The openingsalso reduce the weight of the alignment guide.

The distal portion of the arm or arms functions as an assembly surfacesealing member. Hereinafter the distal portion of the arm isinterchangeably referred to as an “assembly surface seating member”.

Optionally, the assembly surface seating member can be provided by anin-turned lip at or near the end of the arm.

The assembly surface seating member can include one or more surfacefeatures which engage positively with the assembly surface of a headpart. This can ensure that the head part and the assembly surfaceseating member can be assembled with a single stable position relativeto one another. The head part can be held within the alignment guide asa result of engagement of the surface features with the assembly surfaceagainst transverse movement. The bore in the assembly surface of thehead part is aligned with the second axis defined by the axial portionwhen the surface features on the assembly surface are engaged positivelywith the assembly surface of a head part. This ensures that the secondaxis as defined by the axial portion) along which force is applied tothe head part of the prosthesis component is aligned with the first axis(as defined by the bore). An example of a surface feature is a recesswhich can engage the head part around at least part of the externalperiphery of the assembly surface. The recess can be continuous. Therecess could be defined by one or more protrusions which engage theassembly surface of the head part at spaced apart points around the headpart. A surface feature could include one or more protrusions which fitinto corresponding detents formed in the assembly surface.

The assembly surface seating member can be shaped so that it restrictstransverse movement of the head part of the prosthesis component. Forexample, the assembly surface seating member can engage an edge of theassembly surface of the head part. The edge of the assembly surface canbe an outside edge. The outside edge of the assembly surface might be ator close to an outside edge of the head part when the head part isshallow (for example when the depth of the head part is less than halfof its width, as can be the case with the humeral component of ashoulder prosthesis). The outside edge of the assembly surface might beat the interface between the rounded bearing surface of the head partand the assembly surface. The assembly surface might have a chamferportion which is located between the rounded fearing surface and thebore. The outside edge of the assembly surface might be at the interfacebetween the rounded bearing surface of the head part and the chamferportion of the assembly surface. The lip might engage other features onthe assembly surface. For example, the assembly might make use of one ormore protrusions on one of the assembly surface seating member and theassembly surface of the head part and one or more detents on the otherof the assembly surface seating member and the assembly surface.

The assembly surface seating member can include a series of surfacefeatures which can engage with the assembly surfaces of a plurality ofdistinct head parts having different sizes. For example, an assemblysurface seating member can be provided with a series of recesses, eachof which is configured to engage a respective head part. For example,when the assembly surfaces on the head parts are circular in outline,the recesses can be concentric, with each one shaped as part or thewhole of a circle. The head part can be restrained against translationrelative to the assembly surface seating member when it is engaged withits respective recess.

It is also envisaged that the surface of the assembly surface seatingmember might be free of interruptions so that the assembly surface of ahead part can translate across the surface of the assembly surfaceseating member. The surface could be essentially planar. The surfacemight be profiled so as to promote centring of the head part. Forexample, it might be concave with a centre which is aligned with thecentre of the bore in a head part when properly positioned on thesurface. The head part can be located appropriately on the assemblysurface seating member as a result of engagement between the bearingsurface seating member and the bearing surface, causing the assemblysurface to translate on the surface of the assembly surface seatingmember until it is properly centred. An alignment guide having thesefeatures might accommodate a plurality head parts having differentsizes. For example, head parts with different sizes, and therefore withdifferently sized assembly surfaces can contact different regions of thesecond seating member. The different regions can be arrangedconcentrically when the head parts are circular.

It can be preferred that the assembly surface seating member engages theassembly surface of the head part at least three spaced apart points.The assembly surface seating member can be provided in multiple sectionson respective arms which extend separately from the axial portion, witheach section of the assembly surface seating member engaging theassembly surface at a respective point around the assembly surface. Forexample, when the alignment guide provides an in-turned lip at thedistal end of a narrow arm, it can be preferred that there are at leastthree such arms whose in-turned lips can engage the assembly surface ofthe head part at three spaced apart points around the assembly surface.Sections of the assembly surface seating member can be provided on twoarms where each it section is approximately U-shaped. The two arms canbe positioned next to one another so that the sections of the assemblysurface seating member together extend almost around the entire assemblysurface of the head part apart possibly from small breaks at the gapsbetween the arms. The assembly surface seating member can have a slotformed in it which is open to one side so that it is approximatelyU-shaped. These arrangements can help to ensure that the head part ofthe prosthesis component is located positively relative to the alignmentguide.

In some constructions, the alignment guide an include a deployableretainer which, when deployed, can help to retain the head part withinthe space between the bearing surface seating member and the assemblysurface seating member. The retainer can be deployed to close at leastpartially the opening through which the head part is inserted into thespace between the seating members. The retainer can slide betweendeployed and retracted positions. When the space between the seatingmembers is shaped to receive a generally spherical head part, theretainer can be shaped so as to follow a generally spherical contour. Aretainer which can follow a generally spherical contour can be retractedto a position in which it is in a surface-to-surface facing relationshipwith an adjacent wall portion of the alignment guide.

An alignment guide can include a deployable retainer in addition to abearing surface sealing member which is formed from a resilientlydeformable material and engages the head part of the prosthesiscomponent with a press fit.

In some constructions, the alignment guide can have stand surfaces whichenable it to be positioned on a surface while a head part is positionedwithin the alignment guide. For example, when an alignment guide has twoor more pivoting arms with which it can be engaged with a head part andthen used to move or otherwise manipulate the head part, each of thearms can have stand surfaces which enable the guide to be positioned ona flat surface with each of the stand surfaces in contact with thesurface. This can be convenient during preparatory steps prior to asurgical procedure, minimising the need for a user to contact surfacesof the head part directly, and avoiding the need for the head partitself to be placed in contact with the flat surface.

It will often be preferred that some or all of the components of thealignment guide are made from a polymeric material. This has theadvantage of light weight compared with components made from metals.Components made from polymeric materials can be manufacturedconveniently using moulding techniques. Components made from certainpolymeric materials can lie less likely to damage (for example byscratching) a polished surface such as a prosthesis component bearingsurface compared with components made from a metal.

The invention also provides a method of assembling an orthopaedic jointprosthesis, which comprises:

a. providing an assembly according to the invention together withanother part of the orthopaedic joint prosthesis component which has aspigot which can be received in the bore in the head part.

b. mounting the head part of the orthopaedic joint prosthesis componentwithin the alignment guide so that the distal portion of the arm isengaged with the assembly surface of the head part, and the axialportion engages, either directly or indirectly, the bearing surface ofthe head part.

c. locating the spigot on the other part of the prosthesis component inthe bore in the head part, and

d. applying an impaction force to the head part through the axialportion, in which the impaction force applied to the second axis asdefined by the axial portion is directed along the first axis as definedby the bore in the head part.

The invention also provides a method of implanting an orthopaedic jointprosthesis, which comprises:

a. providing an assembly according to the invention together withanother part of the orthopaedic joint prosthesis component which has aspigot which can be received in the bore in the head part.

b. mounting the head pan of the orthopaedic joint prosthesis componentwithin the alignment guide so that the distal portion of the arm isengaged with the assembly surface of the head part, and the axialportion engages, either directly or indirectly, the bearing surface ofthe head part.

c. locating the spigot on the other part of the prosthesis component inthe bore in the head part, and

d. applying an impaction force to the head part through the axialportion, in which the impaction force applied to the second axis asdefined by the axial portion is directed along the first axis as definedby the bore in the head part.

in which the said other part of the orthopaedic joint prosthesiscomponent is implanted in a patient's bone prior to the step of applyingan impaction force.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a side view of a modular femoral component of a hip jointprosthesis.

FIG. 2 is an isometric view of a head part of a femoral component of ahip joint prosthesis.

FIG. 3 is an isometric view of an orthopaedic joint prosthesis assemblycomprising a modular femoral component of a hip joint prosthesis and analignment guide for use in assembling the femoral component.

FIG. 4 is an isometric view of the alignment guide shown in FIG. 3 andthe head part of the femoral component, with the arms of the alignmentdevice in an open position.

FIG. 5 is a cross-section of the alignment guide shown in FIG. 3 withthe arms in an open position;

FIG. 6 is a cross-section of the alignment guide shown in FIG. 3together with the femoral component.

FIG. 7 is an isometric view from above of an alignment guide and thehead part of a femoral component of a hip joint prosthesis.

FIG. 8 is an isometric view from below of the alignment guide and headpart shown in FIG. 7.

FIG. 9 is an isometric view of an orthopaedic joint prosthesis assemblycomprising a modular femoral component of a hip joint prosthesis and thealignment guide shown in FIG. 7.

FIG. 10 is an isometric view of another orthopaedic joint prosthesisassembly comprising a modular femoral component of a hip jointprosthesis and the alignment guide.

FIG. 11 is a cross-section through the assembly shown in FIG. 10.

FIG. 12 is an isometric view of another orthopaedic joint prosthesisassembly comprising a modular femoral component of a hip jointprosthesis and the alignment guide.

FIG. 13 is a cross-section through the assembly shown in FIG. 12.

FIG. 14 is an isometric view of another orthopaedic joint prosthesisassembly comprising a modular femoral component of a hip jointprosthesis and the alignment guide.

FIG. 15 is a cross-section through the assembly shown in FIG. 14.

FIGS. 16a to 16c show isometric views of another orthopaedic jointprosthesis assembly comprising a modular femoral component of a hipjoint prosthesis and the alignment guide.

FIGS. 17a and 17b shows cross-sectional views through the longitudinalaxis of the assembly shown in FIGS. 16a and 16 b.

FIG. 18 is an isometric view of the assembly shown in FIG. 16 with animpaction rod inserted into the alignment guide.

FIG. 19 is an isometric view of another orthopaedic joint prosthesisassembly comprising a modular femoral component of a hip jointprosthesis and the alignment guide.

FIG. 20 is an isometric view of the assembly shown in FIG. 19 with animpaction rod inserted into the alignment guide.

FIG. 21 is an isometric view of another orthopaedic joint prosthesisassembly comprising a modular femoral component of a hip jointprosthesis and the alignment guide.

FIG. 22 is an isometric view of the assembly shown in FIG. 21 with animpaction rod inserted into the alignment guide.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings. FIG. 1 shows a femoral component 1 of a hipjoint prosthesis which includes a head part 2 and a stem part 3. Thestem part 3 is configured to be implanted in the intramedullary cavityof a patient's femur.

The head part 2 has a smooth outer bearing surface 6 which is intendedto articulate with a corresponding joint surface. The correspondingjoint surface will usually be provided by an acetabular component of thehip joint prosthesis. However, it might be that a head part might beintended to articulate with a corresponding joint surface provided bythe patient's natural tissue. The outer bearing surface has a generallyspherical shape, which is truncated to define an assembly surface 8. Atapered bore 5 is formed in the head part, extending inwardly from theassembly surface, perpendicular to the assembly surface. The first axis7 of the head part extends perpendicular to the assembly surface and isdefined by tapered bore 5.

The assembly surface is at one end of the axis 7 and the centre of thebearing surface is at the opposite end of the axis. The assembly surfacesurrounds the bore. The assembly surface is generally planar, defined bya straight line when the head part is viewed in cross-section. The fiatportion extends annularly around the opening to the bore in the headpart. The flat portion is planar, containing the plane of the opening tothe bore in the head part.

The stem part 3 includes a tapered spigot 4. The tapered spigot on thestem part and the tapered bore in the head part are configured so thatthey can form a self-locking taper lock when the head and stem parts areassembled. Preferably, the angle between the tapered surface of each ofthe spigot and the bore and the longitudinal axis of the spigot and thebore (when the part is viewed in cross-section) is about 1.4° to about1.5°.

FIG. 2 shows another head part of a femoral component of a hip jointprosthesis. The assembly surface includes a planar portion 8 surroundingthe bore 5, which lies in a plane which is perpendicular to the axisdefined by the bore. The assembly surface also includes an annularchamfer portion 8 a which is located between the planar portion 8 andthe bearing surface 6 of the head part. The chamfer portion extendsaround the head part, and which is inclined to the plane which isdefined by the opening to the bore in the head part when the head partis viewed in cross-section. The assembly surface could include anannular portion which is curved, generally in a convex sense, when thehead part is viewed in cross-section. When the assembly surface has achamfer portion or a curved portion without a planar portion, there willbe a narrow ridge surrounding the opening to the bore 5.

FIG. 3 shows an orthopaedic joint prosthesis assembly which includes thefemoral component 1 shown in FIG. 1 and an alignment guide 10 for use inassembling the stem and head parts 2, 3 of the femoral component.

The alignment guide 10 comprises an axial portion in the form of a shaft11 and arms 16 a, 16 b extending therefrom. The shaft has first andsecond portions 12, 13 and carries an impaction tip 15 which can contactthe bearing surface of a head part of a femoral prosthesis component,functioning as a bearing surface seating member. The shaft defines asecond axis X2 (as shown in FIG. 5). The diameter of the first portion12 of the shaft is greater than that of the second portion 13. A taperedportion 24 of the shaft extends between the first and second portions.In the construction shown in the drawings, the shaft 11 has a bore 14 inthe first portion 12 at its free end in which the end of an impactiondevice can be received through which an impaction force can be appliedto the impaction component. However, the shaft might be acted ondirectly by an impaction device such as a mallet. The surface of theimpaction tip 15 is concave, to match the convex bearing surface of thehead part 2 of the femoral component.

Each arm 16 a, 16 b includes a collar portions 18 a, 18, a neck portion19 a, 19 b and a head portion 20 a, 20 b. Each collar portion 18 a, 18 bcomprises a pair of curved projections 29 a. 29 a′; 29 b, 29 b′ whichhave a partially cylindrical profile and are dimensioned to fit around aportion of the exterior surface of the first portion 12 of the shaft 11.The two arms 16 a, 16 b are pivotally connected to one another at thecollar portions 18 a, 18 b. The pivotal connections are achieved inproviding a lug 28 a, 28 b on one of the curved projections 29 a′, 29 bof the collar portions 18 a, 18 b of the arms 16 a, 16 a, and anaperture 30 a, 30 b on the other curved projection 29 a, 29 b′ of therespective collar portions 18 a, 18 b of the arms 16 a, 16 b. A lug 28a, 28 b cooperates with a respective aperture 30 a, 30 b to define apivot point 31 a, 31 b.

The arms 16 a, 16 b are substantially the same and have a curved,bowl-shaped, contour such that when they abut each other in a closedposition, the two arms 16 a, 16 b define an internal cavity or space 17which encloses the femoral head part 2. This is shown in FIG. 4.

The arms 16 a, 16 b are shaped so that the neck 19 is circular incross-section and defines a hollow cylinder with a diametersubstantially similar to that of the collar 18.

The distal portion of each of the arms 16 a, 16 b, i.e., the portion ofeach arm remote from the collar portions 18 a, 18, has an in-turned lip22 a, 22 b. When the arms are in their closed position, the lips 22 a,22 b form an almost continuous annular surface which faces towards thecollar 18, with just small discontinuities at the interlaces between thearms. An opening 23 is provided in the end of the alignment guide bysemi-circular cut-outs 27 a, 27 b in the ends of the arms 16 a, 16 b.The opening 23 is larger than the opening of the bore 5 into theassembly surface 8 of the head part 2 so that access to the recess 5 isnot obscured by the lips 22 a, 22 b.

The arms 16 a, 16 b include windows 32 in the side walls of the headportions 20 a, 20 b which reduce the weight of the alignment guide 10.The windows also allow a user to inspect the femoral head part withinthe internal cavity 17.

Each of the neck portions 19 a, 19 b includes a tab 25 with an inclinedsurface 26 which engages and cooperates with the inclined surface 24 ofthe impaction component 11 when the alignment guide 21 is in a first,unactuated position. The tabs 25 are provided internally or the neck 19of the alignment guide 21.

In use, the spigot 4 on the stem part 3 of the femoral component islocated in the bore 5 in the head part 2 and the head and stem parts arepressed together.

The head part is then located in the space 17 while the arms 16 a, 16 bare in their open position, so that the bearing surface 6 is in contactwith the impaction tip 15. The arms are then pivoted to their closedposition in which the lips 22 a, 22 b engage the assembly surface 8 onthe head part 2 so that the head part is located between the impactiontip 15 and the lips 22 a, 22 b. The spigot 4 on the stem part thenextends through the opening 23 provided by the semi-circular cut-outs 27a, 27 b in the ends of the arms. The engagement of the annular surfaceprovided by the lips 22 a, 22 b with the assembly surface determines theorientation of the head part relative to the alignment guide, inparticular so that the first axis XI defined by the bore 5 in the headpart 2 is coincident with the second axis X2 which is defined by theshaft 11 of the alignment guide 10. This ensures that when an impactionforce is applied along the second axis X2 the force is directed alongthe first axis X1.

An impaction tome is applied to the shall 11 through an impaction rodwhich is inserted into the bore 14 in the end of the first portion 12 ofthe shaft. Alternatively, the impaction force could be applied directionto the shaft. The impaction force is transmitted through the shaft tothe head part of the femoral component, which involves translation ofthe shaft relative to the alignment guide 21. The translation of theshaft, with the tapered portion 24 of the shaft in contact with the tabs25 on the internal surfaces of the neck 19 of the alignment guide 21,causes the arms 16 a, 16 b to pivot outwardly, reducing the contactbetween the lips 22 a, 22 b and the assembly surface 8 of the head part.

FIGS. 7 to 9 show an orthopaedic joint prosthesis assembly whichincludes a head part 100 of a femoral component such as that shown inFIG. 2 and an alignment guide 110 for use in assembling the stem andhead parts of a femoral component. The head part 100 of the femoralcomponent can be fitted to a stem part having a tapered spigot at itsproximal end. The head part 100 has a bearing surface 104 and a bore 106in an assembly surface 108. The bore defines a first axis X1. Theassembly surface can include a fiat planar portion which lies in theplane defined by the opening to the bore in the head part, and a chamferportion 108 a between the fiat planar portion and the bearing surface ofthe head part. The chamfer portion extends around the head part which isinclined to the plane which is defined by the opening to the bore in thehead part when the head part is viewed from one side in cross-section.

The alignment guide 110 comprises an axial portion in the form of a hubspar 112. First and second arms 114, 116 extend from the hub spar. Thehub spar is curved with a concave inner surface 118 which defines aspace within it. The hub spar has a socket 132 formed in it in. Thecentre of the socket 132 defines a second axis X2. A sleeve component134 can be provided in the socket as shown in FIG. 9. A shaft 136 canextend through the sleeve component and is a sliding fit therein. Whenthe tool includes a sleeve component, the first and second arms can havesemi-circular cut-outs so that the arms can fit snugly against thesleeve component.

Each of the arms 114, 116 is connected to the hub spar at the first andsecond pivot points 122, 124 so that each of the arms can pivot relativeto the spar. Each of the arms has a tab 126 which can be engaged by auser to move the arm between its deployed position as shown in thedrawings and a retracted position in which each of the arms is pivotedtowards the hub spar.

The hub spar includes first and second extensions 128, 130 beyond thepivot points 122, 124. Each of the extensions has a flat surface 134 atits end.

Each of the arms 114, 116 has a lug 136 mounted on its tab 122. The lugshave inner surfaces 138 and outer surfaces 140. The inner surfaces ofthe lugs are directed towards one another when the arms are in theirdeployed positions (which is the case as shown in the drawings).

The space which is defined by the inner surface 118 of the hub spar 112is sized to receive the head part 100. When the bearing surface 104 ofhead part is shaped as part of a sphere, the concave inner surface ofthe hub spar will also be shaped as part of a sphere. The pivot points122, 124 can be provided on the hub spar at its widest point. The widthof the space which is defined by the hub spar is greatest between thepivot points. The width of the space which is defined between theextensions 128, 130 is less than that between the pivot points.

The hub spar 112 and the arms 114, 116 can be formed from a polymericmaterial, for example by injection moulding. Examples of a suitablepolymeric material include certain polyamides, polyesters, polyolefinsand poly(phenyl sulphones). A suitable material should be capable ofwithstanding conditions to which it is exposed during manufacture(including sterilisation) and use. A suitable material will often beresiliently deformable.

In use, the head part 100 of a femoral prosthesis component can befitted into the alignment guide 110 when the arms 114, 116 are in theirretracted positions. This involves displacing the extensions 128, 130 atthe ends of hub spar 112 outwardly so that the widest part of thespherical bearing surface 104 is positioned beyond the extensions 128,130, generally in line with the pivot points 122, 124. The resilientdeformability of the material of the hub spar means that the head partis retained within the space 120 defined by the hub spar by means of theextensions.

Once the head part has been positioned within the space defined by thehub spar, the arms 114, 116 are pivoted from their retracted positionsto their deployed positions. This can be performed by a user by engagingthe tabs 122, for example with the finger and thumb of one hand. Theinner surfaces 138 of the lugs 136 engage the chamfer surface portion108 a of the head part at diametrically opposite points of the chamfersurface. The head part is then located between the concave surface ofthe hub spar which contacts the bearing surface and the lugs 136 whichcontact the assembly surface, the concave surface of the hub spar andthe lugs being the bearing surface seating member and assembly surfaceseating members respectively.

The outer surfaces 140 of the lugs 136 and the flat surfaces 134 at theends of the extensions are approximately coplanar. The assembly of thehead part 100 and the alignment guide 110 can be placed on a surface(for example a table) with the outer surfaces 140 of the lugs 136 andthe flat surfaces 134 at the ends of the extensions in contact with thetable. The head part 100, including in particular the assembly surface108 a of the head part, is visible for inspection between the lugs andthe extensions. The head part 100 can be manipulated by a user grippingthe alignment guide, including positioning the head part so that thespigot on the stem part of the femoral component is received within thebore 106 in the head part. It is not necessary to contact the bearingsurface of the head part.

The engagement between the hub spar 112 and the lugs 136 against thehead part ensures that the head part is located centrally within thealignment guide with the first axis X1 that is defined by the bore inthe head part being coincident with the second axis X2 defined by thecentre of the socket 132 in the hub spar.

An impaction force can be applied to the head part to achieve a secureconnection between it and the stem part through an impaction shaft 142which extends through the bore in the sleeve component 134. The sleevecomponent ensures that the longitudinal axis of the shaft extendsperpendicular to the plane of the socket 132 in the hub spar, and iscoincident with the second axis X2 defined by the centre of the socket.An impaction force that is directed through the impaction shaft 142 istherefore coincident with the first axis X1 that is defined by the borein the head part. This ensures that when an impaction force is appliedalong the second axis X2 the force is directed along the first axis X1.

FIGS. 10 and 11 show an orthopaedic joint prosthesis assembly whichincludes a head part 200 of a femoral component such as that shown inFIG. 2 and an alignment guide 210 for use in assembling the stem andhead parts of a femoral component. The head part 200 of the femoralcomponent can be fitted to a stem part having a tapered spigot at itsproximal end. The head part 200 has a bearing surface 204 and a bore 206in an assembly surface 208. The assembly surface includes a chamferportion 208 a.

The alignment guide 210 comprises an axial portion in the form of athreaded sleeve 218. The threaded sleeve defines a second axis X2 which,when the head part of an orthopaedic joint prosthesis component ismounted within the alignment guide, is coincident with the first axis X1defined by the bore 206. An arm 212 having proximal and distal ends 214,216 extends in a distal direction from the threaded sleeve 218. Thedistal end 216 of the arm 212 has a U-shaped bracket 220.

A threaded shaft 222 extends through the sleeve. The longitudinal axisof the shaft 222 is coincident with the longitudinal axis of the sleeve218. The thread on the shaft 222 engages the thread in the sleeve 218 sothat the shaft can be advanced and retracted through the sleeve byrotating the shaft relative to the sleeve. The shaft has a first end 224which is closer to the second end 216 of the arm 212, and an oppositesecond end 226.

The shaft 222 has a boss 226 at the end of the shaft which is closer tothe second end 216 of the arm 212. The boss is defined by a shoulder 228on the shaft.

The alignment guide includes a circular seating member 230 which has aconcave surface 232 defined by a part of a sphere on one side. It has arecess 234 on its opposite side which can receive the boss 226 on theend of the shaft. The curvature of the concave surface of the circularseating member corresponds approximately to the curvature of the bearingsurface 204 of the head part 200 so that the head part fits against theseating member. The engagement between the recess on the seating memberand the boss on the end of the shaft means that the seating member canremain stationary in contact with the bearing surface of the head partwhen the shaft is rotated.

The shaft 222 carries a socket member 236 at the second end 226 of theshaft. The socket member can receive the end of an impaction rod throughwhich an impaction force can be applied. An impaction force can beapplied using a mallet or using an instrument such as the one disclosedin EP-A-1707160. The external surface of the socket member 236 is ridgedto facilitate gripping the socket member to twist it and the shaft.

The U-shaped bracket 220 carries a U-shaped seating member 238 which isalso U-shaped. The space between the arms of the seating member is atleast equal to the diameter of the bore 206 in the head part 200. Itwill usually be slightly bigger than the diameter of the bore. Theseating member is made from a polymeric material such as a poly(phenylsulphone). The surface of the seating member which faces the proximalend of the arm is smooth.

In use, the head part 200 of a femoral prosthesis component can befitted into the alignment guide 210 when the threaded shaft 222 isretracted so as to create sufficient space between the circular seatingmember 230 on the end of the shaft and the U-shaped seating member 238at the second end of the arm 212, so that the head part is positioned inthe space between the two seating members 230, 238. The shaft isretracted in this way by rotating it relative to the sleeve 218.

The threaded shaft 222 is then advanced through the sleeve 218 byrotating it relative to the shalt to drive the concave surface 232 ofthe circular seating member into contact with the bearing surface 204 ofthe head part 200. This leads to surface to surface contact between theassembly surface 208 of the head part and the exposed face of theU-shaped seating member 238, The head part becomes centred on theU-shaped seating member as the circular seating member becomes seated onthe bearing surface of the head part by translating across the U-seatingmember due to the action of the circular seating member against thebearing surface. The first axis X1 which is defined by the bore 206 inthe head part then extends through the centre or the circular seatingmember 230 and is coincident with the second axis X2 which is defined bythe sleeve 218.

An impaction face can be applied to the head part to achieve a secureconnection between it and the stem part through the shaft 222. Animpaction force that is directed through the impaction shaft, positionedwithin the skis 218, is therefore coincident with the first axis X1 thatis defined by the bore 206 in the head part 200. This ensures that whenan impaction force is applied to the shaft 222 along the second axis X2the force is directed along the first axis X1.

FIGS. 12 and 13 show an orthopaedic joint prosthesis assembly whichincludes a head part 300 of a femoral component such as that shown inFIG. 2 and an alignment guide 310 for use in assembling the stem andhead parts of a femoral component. The assembly has features in commonwith the assembly which is discussed above with reference to FIGS. 10and 11. The head part 300 of the femoral component can be fitted to astem part having a tapered spigot at its proximal end, the head part 300has a bearing surface 304 and a bore 306 in an assembly surface 308. Theassembly surface includes a chamfer portion 308 a.

The alignment guide 310 includes an axial portion in the form of a plainbore sleeve 318. The plain bore sleeve defines a second axis X2 which,when the head part of an orthopaedic joint prosthesis component ismounted within the alignment guide, is coincident with the first axis X1defined by the bore 306. An arm 312 having proximal and distal ends 314,316 extends in a distal direction from the plain bore sleeve 318. Thedistal end of the arm 314 includes a U-shaped bracket 320.

A shaft 22 extends through the sleeve. The longitudinal axis of theshaft 322 is coincident with the longitudinal axis of the sleeve 318.The shaft 322 can be advanced and retracted through the sleeve. Theshaft has a first end 324 which is closer to the distal end 316 of thearm 312, and an opposite second end 326.

The shaft 322 has a boss 326 at the end of the shaft which is closer tothe second end 316 of the arm 312. The boss is defined by a shoulder 328on the shaft. A spring 329 acts between an end of the sleeve 318 and theboss 326.

The alignment guide includes a three fingered seating member which has aconcave surface 331 defined by a part of a sphere on one side. Theseating member has a central hub 332 and three fingers 333 extendingradially from the hub which provide the concave surface 331. Thecurvature of the concave surface of the circular seating membercorresponds approximately to the curvature of the bearing surface 304 ofthe head part 300 so that the head part fits against the seating member.The seating member has a recess 334 on the side which is opposite to theconcave surface which can receive the boss 324 on the end of the shaft.The engagement between the recess on the seating member and the boss onthe end of the shaft means that the seating member can remain stationaryin contact with the bearing surface of the head part when the shaft isrotated.

The shaft 322 carries a socket member 336 at the second end 326 of theshaft. The socket member can receive the end of rod through which animpaction force can be applied. The socket member 336 is fastened to theend of the shaft by means of a pin 337. An impaction force can beapplied using a mallet or using an instrument such as the one disclosedin EP-A-1707160. The socket member has a series of circumferentialridges which facilitate gripping the socket member to apply an axialthree to the shaft, against the action of the spring 329.

The U-shaped bracket 320 carries a U-shaped seating member 338 which isalso U-shaped. The space between the arms of the seating member is atleast equal to the diameter of the bore 306 in the head part 200. Itwill usually be slightly bigger than the diameter of the bore. Theseating member is made from a polymeric material such as apoly(phenylsulphone). The surface of the seating member which faces thefirst end of the arm is smooth, allowing the head part to translate onthe seating member as describe above.

In use, the head part 300 of a femoral prosthesis component can befitted into the alignment guide 310 when the shaft 322 is retracted soas to create sufficient space between the circular seating member 330 onthe end of the shaft and the U-shaped seating member 338 at the secondend of the arm 312, so that the head part is positioned in the spacebetween the two seating members 330, 338. Retraction of the shaft 322involves pulling it through the sleeve, compressing the spring 329between the end of the sleeve 318 and the boss 326. The shaft can beretracted in this way by gripping the socket member 336. The extent ofmovement of the shaft relative to the sleeve is restricted by means of apin 340 which is located in a bore in the sleeve and extends through alongitudinal slot in the shaft.

The shaft 322 is then released so that it advances through the sleeve318, acted on by the spring 329, so that the circular seating membercontacts the bearing surface 304 of the head part 300. This leads tosurface to surface contact between the assembly surface 308 of the headpart and the exposed face of the U-shaped sealing member 338. The headpart becomes centred on the U-shaped seating member as the circularseating member becomes seated on the bearing surface of the head part bytranslating across the U-seating member due to the action of thecircular seating member against the bearing surface. The first axis XIwhich is defined by the bore 306 in the head part then extends throughthe centre of the circular sealing member 330 and is coincident with thesecond axis X2 which is defined by the sleeve 318.

An impaction force can be applied to the head part to achieve a secureconnection between it and the stent part through the shaft 322. Animpaction force that is directed through the impaction shaft 3222 istherefore coincident with the first axis X1 that is defined by the borein the head part. This ensures that when an impaction force is appliedalong tire second axis X2 the force is directed along the first axis X1.

FIGS. 14 and 15 show an orthopaedic joint prosthesis assembly whichincludes a head part 400 of a Femoral component such as that shown inFIG. 2 and an alignment guide 410 for use in assembling the stem andhead parts of a femoral component. The assembly has features in commonwith the assembly which is discussed above with reference to FIGS. 10and 11. The head part 400 of the femoral component can be fitted to astem part having a tapered spigot at its proximal end. The head part 400has a bearing surface 404 and a bore 406 in an assembly surface 408. Theassembly surface includes a chamfer portion 408 a.

The alignment guide 410 includes an axial portion in the form of athreaded sleeve 418. An arm 412 which has proximal and distal ends 414,416 extends in a distal direction from the sleeve 418. The distal end ofthe arm has a U-shaped slot 420 in the end wall 438. The arm extendsaround an angle of arc measured around an axis extending between theproximal and distal ends of about 185 to 190°. It therefore forms a wallwhich encloses the space between the proximal and distal ends on oneside of the alignment guide. The wall can be provided with openings (notshown) to make a head part located within the alignment guide morevisible. The arm is made from a polymeric material by moulding. Thematerial is capable of resilient deformation.

A threaded shaft 422 extends through the sleeve. The longitudinal axisof the shaft 422 is coincident with the longitudinal axis of the sleeve418. The thread on the shaft engages the thread in the sleeve 418 sothat the shaft can be advanced and retracted through the sleeve byrotating the shaft relative to the sleeve. The shaft has a first end 424which is closer to the proximal end 416 of the arm 412, and an oppositesecond end 426.

The shaft 422 has a 423 Mare extending through it which is open at eachof the first and second ends 424, 426. The shaft has a ridged collar atthe second end which allows the shaft to be gripped in order to rotateit relative to the sleeve 418.

The end wall 428 of the shaft 322 surrounding the open end of the bore423 is shaped so that it presents a generally concave surface which isan annular portion of a sphere. The end wall is then a circular seatingmember. The curvature of the concave surface of the circular seatingmember corresponds approximately to the curvature of the bearing surface404 of the head part 400 so that the head part fits against the seatingmember. The engagement between the recess on the seating member and theboss on the end of the shaft means that the seating member can remainstationary in contact with the bearing surface of the head part when theshaft is rotated.

The space between the arms of the U-shaped slot 420 is at least equal tothe diameter of the bore 406 in the head part 400. It will usually beslightly bigger than the diameter of the bore.

In use, the head part 400 of a femoral prosthesis component can befitted into the alignment guide 410 when the threaded shaft 422 isretracted so as to create sufficient space between the circular bearingsurface seating member provided by the end 428 of the shaft and theassembly surface seating member provided by the end wall 438 at thedistal end of the arm 412. The end wall 438 provides an assembly surfaceseating member for the head part so that the head part is positioned inthe space between the two seating members 430, 438. The shaft isretracted in this way by rotating it relative to the sleeve 418.

The threaded shaft 422 is then advanced through the sleeve 418 byrotating it relative to the shaft to drive the end wall 428 of the shaftinto contact with the bearing surface 404 of the head part 400. Thisleads to surface to surface contact between the assembly surface 408 ofthe head part and the end wall 438 of the second end of the arm. Thehead part becomes centred on the end wall as the circular seating memberbecomes seated on the bearing surface of the head part by translatingacross the U-seating member due to the action of the circular seatingmember against the bearing surface. The first axis X1 which is definedby the bore 406 in the head part is then coincident with the second axisX2 which is defined by the sleeve 418.

An impaction force can be applied to the head part to achieve a secureconnection between it and the stem part through the shaft 322. Animpaction force that is directed through the threaded shaft 412 istherefore coincident with the first axis X1 that is defined by the borein the head part. This ensures that when an impaction force is appliedalong the second axis X2 the force is directed along the first axis X1.

An impaction force can be applied to the head part through an impactionshaft which is inserted through the bore 423 in the threaded shaft 422.The longitudinal axis of the bore in the threaded shall is coincidentwith the second axis X2 as defined by sleeve 418. In turn the secondaxis X2 is coincident with the first axis X1 defined by the bore 406 inthe head part. This helps to ensure that the impaction force is directedalong the first axis X1.

FIGS. 16 to 18 show aid orthopaedic joint prosthesis assembly whichincludes a head part 500 of a femoral component such as that shown inFIG. 2 and an alignment guide 510 for use in assembling the stem andhead parts of a femoral component. The assembly has features in commonwith the assembly which is discussed above with reference to FIGS. 14and 15. The head part 500 of the femoral component can be fitted to astem part having a tapered spigot at its proximal end. The head part 500has a bearing surface 504 and a bore 506 in an assembly surface 508. Theassembly surface includes a chamfer portion 508 a.

The alignment guide 510 includes an axial portion in the form of a plainbore sleeve 518 having a longitudinal axis X2. First and second arms512, 514 extend from the distal portion of the sleeve 518 such that aspace is formed between each arm. The inner surface of each arm has acontact surface, the shape of which substantially corresponds to that ofthe bearing, surface of the head part of an orthopaedic jointprosthesis. This facilitates a nesting fit between the bearing surfaceof the head part and the contact surface of each arm. When the bearingsurface of the head is convex, the contact surface of each arm can beconcave. As shown in FIGS. 16b and 16 c, retention pinch points, can beprovided on the contact surface of each of the first and second arms512, 514. The retention pinch points are preferably located at or abovethe equator of the femoral head component. The retention pinch pointsaid in the stable retention of the femoral head component within thealignment guide and also apply a force to push the femoral headcomponent downwardly towards the assembly surface seating member. Theretention pinch points can take the form of an inwardly protruding rib524 a, 524 b (as shown in FIG. 16a ) or an angled straight face 526 a,526 b (as shown in FIG. 16c ).

FIGS. 17a and 17b show cross sectional views of the assembly shown inFIG. 16a taken along the longitudinal axis X2. FIG. 17a shows theretention of a 36 mm femoral head with a −2.0 offset retained within thealignment guide. FIG. 17b shows the retention of a 36 min femoral headwith a +8.5 offset retained within the same alignment guide. The abilityof the same alignment guide to retain femoral heads with differentoffsets is achieved by the provision of the retentions pinch point 524a, 524 b.

Each arm 512, 514 has a proximal end 516 and a distal end 520. Thedistal end of each arm joins to form a U-shaped bracket 522. TheU-shaped bracket carries a U-shaped assembly surface seating memberwhich is also U-shaped. The space between the arms of the seating memberis at least equal to the diameter of the bore 506. The surface of theseating member which contacts the assembly surface is smooth. Thealignment guide is made from a polymeric material by tor examplemoulding or machining. The material is capable of resilient deformation.

The alignment guide can be provided in a range of sizes, each size ofguide being specific for a particular size of head part. For example,the alignment guide shown is configured for use with a 28 mm diameterfemoral head. In sonic constructions, the head part is mounted within,and packaged with the alignment guide. This is particularly advantageousbecause it minimises the amount of handling of the head part prior toimplantation. This reduces the risk of a breach of sterility of the headpart and any damage to the bearing surface.

As shown in FIG. 16 a, the head part 500 of a femoral prosthesiscomponent can be fitted into the alignment guide 510 by press-fittingthe femoral head between the arms 512, 514. The resilient deformity ofthe material of the arms 512, 514 means that the head part is retainedwithin the space defined between the arms. The head part is positionedsuch that there is surface to surface contact between the assemblysurface 508 of the head part and the inner surface of the U-shapedassembly surface seating member. As shown in FIG. 18, an impaction forcecan be applied to the head part through an impaction shaft 522 which isinserted through the bore of the sleeve 518. The distal end of theimpaction shaft is shaped to correspond to that of the bearing surface.When the bearing surface of the head is convex, the distal end of theimpaction shaft can be concave. The distal end of impaction shaft isbrought into contact with the bearing surface and functions as a bearingsurface seating member. The sleeve defines the orientation of theimpaction shaft relative to the axis which is defined by the bore 506.The sleeve ensures that the longitudinal axis of the impaction shaft iscoincident with the second axis X2 of the sleeve. In turn, the secondaxis X2 is coincident with the first axis X1 which is defined by thebore in the head part. This ensures that when an impaction force isapplied along the second axis X2 the three is directed along the firstaxis X1.

FIGS. 19 and 20 show an orthopaedic joint prosthesis assembly whichincludes a head part 600 of a femoral component such as that shown inFIG. 2 and an alignment guide 610 for use in assembling the stem andhead parts of a femoral component. The assembly has features in commonwith the assembly which is discussed above with reference to FIGS. 16and 17. The outer surface of the distal portion of each arm is providedwith a series of grooves which aid in the handling of the alignmentguide. A depression 614 is provided on the outer surface of the sleeve.The depression 614 is dimensioned for receipt of a user's thumb. Thisalso improves the handling of the alignment guide.

FIGS. 21 and 22 show an orthopaedic joint prosthesis assembly whichincludes a head part 700 of a femoral component such as that shown inFIG. 2 and an alignment guide 710 for use in assembling the stem andhead parts of a femoral component. The assembly has features in commonwith the assembly which is discussed above with reference to FIGS. 16and 17. The sleeve 712 is provided with circumferentially arrangedgrooves on the outer surface, The grooves aid in the handling of thealignment guide. A depression 714 is provided on the outer surface ofthe sleeve. The depression is dimensioned for receipt of a user's thumb,This also improves the handling of the alignment guide.

The assembly surface seating member which engages the assembly surfaceof the head part can be essentially planar as discussed above inrelation to at least some of the devices shown in the drawings. Theassembly surface seating member could be formed with at least oneformation defining at least part of a circle, which can engage theassembly surface around at least part of the periphery of the head part.The formation might be U-shaped so that its shape matches that of theseating member, with the base of the “U” being shaped as a part(especially about half) of a circle. The assembly surface seating membercan be formed with a series of formations (for example at least two orat least three or at least four) which can engage the assembly surfaceson head parts of different sizes. This can be appropriate in relation tothe devices shown in FIGS. 10 and 11, FIGS. 12 and 13, and FIGS. 14 and15.

1-16. (canceled)
 17. An orthopedic joint prosthesis assembly whichcomprises: a. a head part of an orthopedic joint prosthesis component,which has a spherical bearing surface for articulation with acorresponding joint surface, and an assembly surface having a boreformed in it for receiving a spigot on another part of the orthopedicjoint prosthesis, said bore having a first axis that extendsperpendicular to the assembly surface, and in which there is adiscontinuity at an interface between the bearing surface and theassembly surface, the said assembly surface being arranged on a planewhich is parallel to, or contains, a plane which is defined by theopening to the bore in the head part when the head part is viewed fromone side in cross-section, and b. an alignment guide comprising an axialportion defining a second axis coincident with the first axis, such whenan impaction force is applied along the second axis the force isdirected along the first axis, the axial portion comprising: a plainbore sleeve; and a pair of arms that extend from a distal portion of thesleeve such that a space is formed between each arm, wherein each armcomprises a proximal end, a distal end and an inner surface that has acontact surface shaped to substantially correspond to the bearingsurface of the head part of the orthopedic joint prosthesis, such thatwhen the head part is mounted within the guide a nesting fit is formedbetween the bearing surface of the head part and the contact surface ofeach arm, and wherein the distal end of each arm joins to form aU-shaped bracket which carries a U-shaped assembly surface seatingmember configured to engage the assembly surface of the head part of theorthopedic joint prosthesis when the head part is mounted within theguide.
 18. The orthopedic joint prosthesis assembly of claim 17, whereinthe contact surface of each arm is concave.
 19. The orthopedic jointprosthesis assembly of claim 17, wherein a retention pinch point isprovided on the contact surface of each arm.
 20. The orthopedic jointprosthesis assembly of claim 19, wherein the retention pinch pointcomprises an inwardly protruding rib.
 21. The orthopedic jointprosthesis assembly of claim 19, wherein the retention pinch pointcomprises an angled straight face.
 22. The orthopedic joint prosthesisassembly of claim 17, wherein the bore in the assembly surface of thehead part has a diameter, and wherein a space between each arm of theU-shaped assembly surface seating member is at least equal to thediameter of the bore.
 23. The orthopedic joint prosthesis assembly ofclaim 17, wherein an outer surface of the distal portion of each armincludes a series of grooves, and an outer surface of the sleeveincludes a depression dimensioned for receipt of a user's thumb, theseries of grooves and the depression aid in the handling of thealignment guide.
 24. The orthopedic joint prosthesis assembly of claim17, wherein an outer surface of the sleeve includes a series of groovesand also a depression dimensioned for receipt of a user's thumb, theseries of grooves and the depression aid in the handling of thealignment guide.
 25. The orthopedic joint prosthesis assembly of claim17, wherein the alignment guide is made from a polymeric material. 26.The orthopedic joint prosthesis assembly of claim 25, wherein thepolymeric material is capable of resilient deformation.
 27. Anorthopedic joint prosthesis assembly comprising: a part of an orthopedicjoint prosthesis having a spigot a head part of an orthopedic; a jointprosthesis component, which has a spherical bearing surface forarticulation with a corresponding joint surface, and an assembly surfacehaving a bore formed in it for receiving the spigot on another part ofthe orthopedic joint prosthesis, said bore having a first axis thatextends perpendicular to the assembly surface, and in which there is adiscontinuity at an interface between the bearing surface and theassembly surface, the said assembly surface being arranged on a planewhich is parallel to, or contains, a plane which is defined by theopening to the bore in the head part when the head part is viewed fromone side in cross-section; b. an alignment guide comprising an axialportion defining a second axis coincident with the first axis, such whenan impaction force is applied along the second axis the force isdirected along the first axis, the axial portion comprising: a plainbore sleeve; and a pair of arms that extend from a distal portion of thesleeve such that a space is formed between each arm; wherein each arm ofthe alignment guide comprises a proximal end, a distal end and an innersurface that has a contact surface shaped to substantially correspond tothe bearing surface of the head part of the orthopedic joint prosthesis,such that when the head part is mounted within the guide a nesting fitis formed between the bearing surface of the head part and the contactsurface of each arm, and wherein the distal end of each arm joins toform a U-shaped bracket which carries a U-shaped assembly surfaceseating member configured to engage the assembly surface of the headpart of the orthopedic joint prosthesis when the head part is mountedwithin the guide.
 28. A method of assembling an orthopedic jointprosthesis, which comprises: a. providing an assembly comprising: a partof an orthopedic joint prosthesis having a spigot; a head part of anorthopedic joint prosthesis component, which has a spherical bearingsurface for articulation with a corresponding joint surface, and anassembly surface having a bore formed in it for receiving the spigot,said bore having a first axis that extends perpendicular to the assemblysurface, and in which there is a discontinuity at an interface betweenthe bearing surface and the assembly surface, the said assembly surfacebeing arranged on a plane which is parallel to, or contains, a planewhich is defined by the opening to the bore in the head part when thehead part is viewed from one side in cross-section; an alignment guidecomprising an axial portion defining a second axis coincident with thefirst axis, such when an impaction force is applied along the secondaxis the force is directed along the first axis, the axial portioncomprising: a plain bore sleeve; and a pair of arms that extend from adistal portion of the sleeve such that a space is formed between eacharm, wherein each arm of the alignment guide comprises a proximal end, adistal end and an inner surface that has a contact surface shaped tosubstantially correspond to the bearing surface of the head part of theorthopedic joint prosthesis, such that when the head part is mountedwithin the guide a nesting fit is formed between the bearing surface ofthe head part and the contact surface of each arm, and wherein thedistal end of each arm joins to form a U-shaped bracket which carries aU-shaped assembly surface seating member configured to engage theassembly surface of the head part of the orthopedic joint prosthesiswhen the head part is mounted within the guide; and b. mounting the headpart of the orthopedic joint prosthesis component within the alignmentguide so that the distal portion of each arm is engaged with theassembly surface of the head part and the contact surface of each armcontacts the bearing surface of the head part, c. locating the spigot onthe other part of the prosthesis component in the bore in the head part,and d. applying an impaction force to the head part through the axialportion, so that when the impaction force is applied along the secondaxis the force is directed along the first axis.